The present invention relates to a method for producing a semiconductor device, and more particularly to such a method for producing a semiconductor device capable of diffusing a heavy metal such as gold, platinum or the like at a high concentration in a lattice of a semiconductor crystal layer, the heavy metal acting as a life time killer.
Conventionally, to improve the switching velocity of a semiconductor device such as transistor, diode or the like, the way wherein a heavy metal such as gold, platinum or the like is diffused in a silicon (silicon) crystal substrate so as to reduce the life time of minority carriers is widely employed. Introduction of, for example, gold into a silicon crystal structure causes the gold atom to serve as a trap which assists in recombination of the minority carriers thereby shortening the life time thereof.
Diffusion of gold atoms in the crystal lattice is very rapidly achieved. Specifically, a diffusing treatment at about 1000.degree. C. for several minutes allows the gold atoms to be uniformly delivered at a concentration which is equal to a solid solubility of gold in silicon at about 1000.degree. C., taking the same effect as that exhibited with such treatment infinitely conducted. However, the solubility of gold in silicon greatly depends on the temperature; for instance, the concentration of gold at 1150.degree. C. in a stable state is 5.times.10.sup.16 /cm.sup.3 while it decreases to 2.times.10.sup.16 /cm.sup.3 with a drop of temperature to 1050.degree. C. An excess amount of gold resulting from the decrease in concentration, or substantially half of the total amount of gold contained in silicon crystal at 1150.degree. C. goes out of the crystal lattice and is scatteredly separated in such a state as to come into fine masses and is deposited in silicon crystal. The deposited gold is electrically inert and no longer assists in recombination of the minority carriers. Accordingly, very rapid cooling is needed to maintain the effective concentration of gold in the crystal lattice which is obtained at a high temperature. This is because the rapid cooling will not give gold a sufficient time to diffuse and hence a high gold concentration can be maintained in the crystal lattice.
A conventional method for diffusing gold in a silicon crystal substrate is effected in such a manner as shown in FIG. 4. Specifically, a semiconductor wafer 6 formed with a semiconductor circuit such as transistor on the front side thereof and covered with a gold film on the reverse side thereof is placed on a quartz boat 15; and the wafer on the boat 15 is loaded into and taken out manually from a chamber 17 of a lateral-type furnace at a velocity of about 3 to 20 cm/second. The furnace has such a heater surrounding it. The reason why it is mannually conducted is that an autoloader for conveying the quartz boat cannot follow the velocity needed for the above-mentioned rapid cooling.
Recently, however, as the diameter of a wafer increases to improve the productivity of semiconductor devices, the quartz boat is also up-scaled in length as well as in width. Accordingly, the boat becomes too heavy for an operator to move it. While the velocity for conveying the boat cannot be controlled accurately. Further, there is another problem that the operation of taking the boat out from the chamber is very dangerous because the temperature immediately after the semiconductor wafer and quartz boat is taken out is very high.
Moreover, the quartz boat is stopped once in the neighborhood of the exit of the chamber in the takeout operation, and the temperature of the chamber in the neighborhood of the exit is substantially low, while there exists an isothermal zone at a portion of more than 70 cm from the exist as can be seen from FIG. 5 showing a temperature distribution in the chamber of the lateral-type furnace. Therefore, if an up-scaled quartz boat (for example, 67.3 cm in length B.sub.2 ; whereas the length B.sub.1 of a conventional boat of usual size is 30.5 cm) is stopped at 15 cm inside from the exit, the wafers in the inner side are located near the high temperature zone and hence cannot be rapidly cooled. This results in a problem that the above-mentioned object of rapid cooling cannot be attained, and the quality of wafer becomes instable depending on the loading location.
Thus, an object of the present invention is to provide a producing method which is capable of efficiently diffusing a heavy metal in crystal lattice, as a life time killer such as gold or the like at a high concentration while being applicable to a semiconductor wafer having an enlarged diameter.